In the display skill field, Liquid Crystal Display (LCD), Organic Light Emitting Diode (OLED) and other panel displays have been gradually replaced the CRT displays.
With the development of panel displays, the high resolution and low power consumption panels has constantly been demanded. The Low Temperature Poly-silicon (LTPS) Thin Film Transistor (TFT) draws the attentions of the industry related to the high resolution Active Matrix Liquid Crystal Display (AMLCD) and Active Matrix Organic Light Emitting Display (AMOLED) to be considered with great application value and potential. Compared with amorphous silicon (a-Si), the LTPS TFT has higher carrier mobility, which can reach up from dozens to hundreds cm2/VS. The response speed of the element is fast, and the stability is good, which can satisfy the demands of the high resolution AMLCD and AMOLED. The LTPS TFT cannot only be the pixel switch but also can be employed to construct the peripheral driving circuit to realize circuit integration.
The LTPS is a branch of the poly-Si technology. The reason why the polysilicon material possesses higher electron mobility is due to the polysilicon structure of the polysilicon itself. Compared with the amorphous silicon of having high defect concentration and being highly disordered, the polysilicon is constructed by a plurality of ordered crystalline grains.
In prior arts, the polysilicon thin film can be obtained by Chemical Vapor Deposition (CVD); also can be obtained by implementing annealing process to the amorphous silicon to be decrystallized, and the common methods are: Solid Phase Crystallization (SPC), Metal-Induced Crystallization (MIC), Excimer Laser Annealing (ELA) and etc.
At present, the main factor of restricting the polysilicon TFT element property is the grain boundary among the polysilicon crystalline grains. A large amount of defects exist at the grain boundary. The grain sizes and distributions of the polysilicon thin film manufactured by the aforesaid various methods are different, and the numbers of the grain boundary defects are quite different, too.
Lots of methods have been proposed to focus how to improve the quality of the polysilicon thin film in the research literatures. Some researches have pointed out that the silicon self-ion implantation can be used to make the polysilicon thin film be decrystallized, and then the recrystallization is performed to reduce the crystal nucleus density and thus, enlarge the grain size after the recrystallization.
R. Reif and J. E. Knott (Electronics Letters, Vol. 17 No. 17, 1981) has already proved that with the silicon ion of high volume (such as 3×1015/cm2) to be implanted in the polysilicon thin film, most crystal structures of the polysilicon thin film can be decrystallized. Due to the tunnel effect of the ion implantation, only few crystal nucleuses are left. These crystal nucleuses grow to be large size grains in the following SPC recrystallization.
N. Yamauchi, etc (IEEE Electron Device Letters, Vol. 11, No. 1, 1990 and Journal of Applied Physics, vol. 75, pp. 3235-3257, 1994) utilizes the polysilicon obtained by the aforesaid method to manufacture TFTs. With the enlarged grain size, the mobility of the element increases, and the subthreshold swing is cliffy. However, in the meantime, the literature also reveals that the uniformity of the small size elements seriously descends because of the random distribution of the large grain sizes.
The above literatures indicates that the silicon self-ion implantation of high volume can decrystallize most of the polysilicon. The decrystallization makes the crystal nucleus density decreased and the large size grains are obtained after recrystallization. The enlarged grains makes the grain boundary defects decreased, and the element property be improved. Nevertheless, this method needs silicon ion implantation of large volume on one hand, and one more crystallization annealing process is required on the other hand. Moreover, the element uniformity issue exists.
To this day, the effect of silicon ion implantation of lower volume which will not cause the decrystallization has not been researched and applied to polysilicon.